Champneys worked as Deputy District Attorney for Mayor's Court from 1824 to 1830. He was elected as a Democratic member of the Pennsylvania House of Representatives in 1825 and 1828. He was appointed Deputy Attorney General for Lancaster County by Governor David R. Porter and served from 1830 to 1833. He served as President Judge of the Second Judicial District in 1839. He was elected to the Pennsylvania Senate for the 6th district and served from 1843 to 1845. He was appointed state Attorney General by Governor Francis R. Shunk and served from 1846 to 1848.
With the onset of the Civil War, he switched parties. In 1863, he served again in the Pennsylvania House of Representatives for Lancaster County. He served again in the Pennsylvania Senate for the 16th district from 1864 to 1865 and for the 17th district from 1865 to 1866.
Brubaker, Henry Clay; Landis, Charles Israel; Eshleman, George Ross; Arnold, Issac Clinton (Oct 6, 1913). "Legal Miscellany: Benjamin Champneys". Lancaster Law Review. XXX (49): 390–1.